This invention relates to the field of organic light emitting diodes. More particularly, it relates to electroluminescent devices which contain fluorescent organic-inorganic hybrid materials.
There is much interest in the field of organic light emitting diodes (OLED""s) as electroluminescent devices for flat panel displays. The requirements that must be met before OLED""s can be used in flat panel displays are: (1) high brightness (100 cd/m2 for indoor use), (2) satisfactory color saturation, (3) high efficiency (which relates to low operating voltage (less than about 15V) and current, and (4) long lifetime (greater than about 50,000 hours).
Many of these requirements cannot be met because of the poor electrical transport properties of most electroluminescent organic materials. For example, charge conduction for these materials requires electrical fields in the range of 3-5 MV/cm. These high electric fields increase power requirements and reduce the lifetime of the device (e.g., through electromigration). In addition, difficulty in balancing the transport of electrons and holes through the device, often results in reduced efficiency.
Recent progress in flat panel displays has led to the development of organic-inorganic films for electroluminescent devices. In these devices, a thin film of an organic-inorganic compound emits visible light when current is supplied to the film. The illumination occurs at a characteristic wavelength of the organic compound when there is a transition from an excited state to the ground state.
U.S. Pat. No. 5,783,292 to Tokito et al. (Tokito) discloses an organic-inorganic electroluminescent device that contains a mixture of inorganic and organic compounds. The organic-inorganic mixtures are deposited on a substrate by vacuum deposition to form thin films. The inorganic-organic thin films can be used in an electroluminescent device for hole transport, electron transport and/or emission layers.
There are many problems with using the organic-inorganic mixtures disclosed in the Tokito patent as emission layers. For example, wide bandgap insulating inorganic materials, such as metal fluorides, metal oxides and metal chalcogenides, are used in which the organic compounds are randomly dispersed. These insulating inorganic materials prevent charge conduction to the organic compounds, resulting in higher power requirements and lower intensity fluorescence. In addition, the organic compound is randomly dispersed in the inorganic compound. Hence, individual organic molecules or clusters of molecules can form in the thin film. The random orientation and clustering of the organic molecules increases the variability of fluorescence and inhibits charge transport. Charge conduction problems can be addressed by using very thin emitting layers (less than 500 Angstroms), however this requires very tedious control of layer deposition and makes devices more prone to defects such as shorts and electrical breakdown. Furthermore, the high vacuum deposition method used in the Tokito patent to deposit the organic-inorganic thin films is very expensive for mass production.
It is an object of the present invention to provide an electroluminescent device containing an organic-inorganic emitting layer that is not random, but orients itself into a predictable arrangement. This reduces the variability of fluorescence and increases charge conduction between the inorganic and organic components. As a result lower power requirements, reduced device degradation, and greater intensity of fluorescence are achieved. Also, thicker film layers can be deposited, resulting in a device that is less prone to shorts and electrical breakdown.
It is a further object of the present invention to provide an electroluminescent device containing an organic-inorganic material that is readily deposited on a substrate by simple and inexpensive methods.
The present invention relates to an electroluminescent device comprising an anode, a cathode and an emitting layer. The emitting layer comprises a self-assembling organic-inorganic hybrid material comprising an organic component and an inorganic component. The organic component comprises a dye that fluoresces in the visible range. In addition, an optically inert component can replace a portion of the organic dye component, resulting in an organic-inorganic hybrid material that has a higher intensity fluorescence than a hybrid material that is fully loaded with the organic dye.